val versus var

In Java, variables can be declared final. Final variables can be assigned only once:

final String s = "Hi";
s = "Bye"; // Doesn't work

Kotlin urges you to use immutable data as much as possible. Final variables in Kotlin are simply val:

val s = "Hi"
s = "Bye" // Doesn't work

If you do have a case in which you would like to reassign a variable, use var instead:

var s = "Hi"
s = "Bye" // Works now